In electrolytic cells for the production of hydrogen and oxygen, such as those of the bipolar and unipolar type, an aqueous caustic solution is electrolyzed to produce oxygen at the anode and hydrogen at the cathode with the overall reaction being the decomposition of water to yield hydrogen and oxygen. The products of the electrolysis are maintained separate by use of a membrane/separator. Use of amorphous metals/metallic glasses and nanocrystalline materials as electrocatalysts for the hydrogen and oxygen evolution reaction are known. The terms "amorphous metal" or "metallic glass" are well understood in the art and define a material which contains no long range structural order but can contain short range structure and chemical ordering. Henceforth, in this specification and claims both terms will be used as being synonymous and are interchangeable. The term "nanocrystalline" refers to a material which possesses a crystallite grain size of the order of a few nanometers, i.e. the crystalline components have a grain size of less than about 10 nanometers. Further, the term "metallic glass" embraces such nanocrystalline materials in this specification and claims.
Numerous nickel-chromium alloys containing additives of other metals or metalloids are known wherein the chromium is provided to enhance corrosion resistance. However, such materials have limited utility in electrocatalysis.
The electrocatalytic behaviour of alloys made by combinations of the two elements Mo and Co to a Ni-base metallic glass have not been reported, although the additions of both Co and Mo to crystalline Ni have shown improved catalytic performance. The reason might be that Mo, when combined with Ni in a crystalline alloy is unstable in alkaline solutions and such alloys undergo preferential dissolution of the Mo constituent (8).
In an electrolysis application, not all of the current which is passed through the cell during electrolysis is utilized in the production of hydrogen and oxygen. This loss of efficiency of the cell is often referred to as the cell overpotential required to allow the reaction to proceed at the desired rate and is in excess of the reversible thermodynamic decomposition voltage. This cell overpotential can arise from: (i) reactions occurring at either the cathode or the anode, (ii) a potential drop because of the solution ohmic drop between the two electrodes, or (iii) a potential drop due to the presence of a membrane / separator material placed between the anode and cathode. The latter two efficiencies are fixed by the nature of the cell design while (i) is directly a result of the activity of the electrode material employed in the cell including any activation or pretreatment steps. Performance of an electrode is then directly related to the overpotentials observed at both the anode and cathode through measurement of the Tafel slope and the exchange current density (hereinafter explained).
Superior electrode performance for the electrolysis of water may be achieved by the use of platinum group metals, alloys and compounds. However, it is desirable to obtain an alloy free of any platinum group metal constituents because of the relatively high cost of all of the platinum group metals. A desirable alternative would then be an alloy comprised of more economical constituents which would still provide the same operating characteristics of a low voltage, high current cell behaviour corresponding to the evolution of hydrogen or oxygen while being electrochemically stable in the alkaline solution.